I have read that it is best to let it run on battery now and then and recharge when 2/3 used. If you just keep it plugged in, actually no problem. The battery will just die sooner, but no matter to you, since you have it plugged in.
I did that with my last laptop and used the battery as a power supply if the power went off (uninterrupted power supply)
Richard is right it will die sooner than it normally would if not used regularly.